Ovens are used to preheat a web of material during the manufacture and forming of products from sheets or webs of plastic material into groups of plastic thin-walled articles. Typically, the oven forms a part of a thermoforming machine having a thermoforming press. A number of different articles can be formed from a sheet or web of plastic material as it is fed from a storage roll. Accordingly, a thermoforming press produces a large quantity of molded articles by intermittently passing a progressing succession of adjacent sections, or shot lengths of web, into the press, after which the web is stamped or formed.
A typical prior oven construction consists of an elongate oven having open leading and trailing ends. Heating elements inside the oven, for example, resistance heaters, operate to heat successive portions of a sheet as it is intermittently delivered into the thermal forming press at a desired molding temperature. However, the ability to properly heat a continuous thermoformable plastic sheet or web of material to a desired temperature depends upon the amount of energy transferred to the sheet, which is, in part, dependent upon the amount of time that the web passes through the oven. Typical oven constructions utilize an oven body having a finite length. Therefore, in order to tailor heat delivery to a thermoformable plastic sheet prior to feeding the sheet into a thermoforming press requires an adjustment of the stationary time that the web sits within the press and oven.
One problem encountered when attempting to adjust the heat delivery to a thermal forming sheet passing through a finite length oven is the necessity of tailoring the oven length to the particular application, or adjusting the stop time during which the thermoforming press has locked the web in a stationary position. In normal operations, it is desirable to increase production rate. Therefore, it is desirable to operate a thermoforming press in as short a cycle time as is physically possible by the constraints of the press operation and web deformation between pairs of interlocking dies. Therefore, it is desirable to tailor heat delivery to the web through some other means. One possibility is to controllably adjust the heat output from each of the thermal resistance elements carried within an oven. However, such an attempt at heat delivery requires careful monitoring of heat being delivered and time during activation for each of the elements. Hence, a complex control scheme is needed to track and target heat and delivery values for each of the elements in an oven.
Although control systems are known for controlling heater elements within a heater oven of a thermoforming line, the ability to precisely realize desired temperatures within a heater oven when heating thermoformable plastic material is particularly desirable in order to achieve high production rates and uniformity of product when thermoforming articles from a heated thermoformable plastic material.